Use assert_separately to speed up on test-all

This creates large string and it pressure GC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39932 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_syntax.rb

@@ -3,11 +3,13 @@ require_relative 'envutil'
 
 class TestSyntax < Test::Unit::TestCase
   def test_syntax
+    assert_separately %w[--disable-gem], __FILE__, __LINE__, <<-'eom'
     assert_nothing_raised(Exception) do
       for script in Dir[File.expand_path("../../../{lib,sample,ext,test}/**/*.rb", __FILE__)].sort
         assert_valid_syntax(IO::read(script), script)
       end
     end
+    eom
   end
 
   def test_must_ascii_compatible